1. Technical Field
The present invention relates to a signal measuring apparatus and a test apparatus. More particularly, the present invention relates to a measuring apparatus that measures information concerning a distribution of phase differences between a first input signal and a second input signal and to a test apparatus that tests a device under test.
2. Related Art
As a semiconductor circuit, a device such as a DDR-SDRAM or the like that outputs a data strobe signal (DQS) in sync with a data signal (DQ) is known. A rear circuit that receives the data signal can reduce transmission degradation caused by jitter of the data signal by acquiring the data signal with a timing of the data strobe signal.
However, when a phase difference between the data strobe signal and the data signal falls outside a prescribed allowable range, there are cases where the rear circuit cannot correctly acquire the data signal. Therefore, in the testing of such a device, the phase difference between the data strobe signal output by the device under test and the data signal is detected and a judgment is made as to whether the phase difference is within the prescribed allowable range.
For example, the phase difference can be detected by obtaining the difference through measurement of both the phase of the data strobe signal and the phase of the data signal by the measuring apparatus. The phase of each signal can be obtained by sampling the signal at every cycle of the signal with a strobe timing in which a phase is sequentially changed and detecting the strobe timing at which the logic value of the signal changes.
However, the data strobe signal and the data signal each contain jitter, and therefore the phase difference between the signals is not constant. Because of this, in a case where the judgment is made as to whether the phase difference between the data strobe signal and the data signal is within the prescribed allowable range, the measuring apparatus measures the phase difference a plurality of times and makes a judgment as to whether all of the phase differences are within the allowable range.
However, there are cases where sufficient analysis of the phase difference between the data strobe signal and the data signal cannot be achieved by just making a judgment as to whether each of the measured phase differences are within the prescribed allowable range. For example, from the thus achieved judgment result, it is difficult to analyze what type of distribution the phase difference has.
A procedure in which a judgment is made as to whether the plurality of phase differences are within the allowable range is repeated while sequentially changing the allowable range to obtain the distribution of the phase differences. In such a procedure, however, the measurement time becomes remarkably long because the phase differences are measured a plurality of times every time the value of the allowable range is changed.